In present day signal generation and processing systems, it is often necessary to produce a signal whose frequency can be varied or swept over a prescribed range. Usually, this involves the need for accurate tracking of signal frequency to a prescribed sweep control input. In many signalling applications, such as radar, spectrum analyzers, etc. the requisite sweep must be linear with time, i.e. a linear frequency ramp, to cover the frequency range of interest. Typically, a voltage controlled oscillator (VCO) is the elementary circuit component that has been used for accomplishing this task since, ideally, it produces an output frequency that is directly related to an input control voltage.
Unfortunately, even though it is possible to generate an input voltage the magnitude of which can be varied with precision, the components of which a voltage controlled oscillator is comprised are subject to environmental conditions, aging and drift, so that, in fact, there may be some degree of offset from the intended output frequency for a specified control voltage. To compensate for this deviation in intended output frequency, there have been proposed a variety of schemes for linearizing such controlled oscillators, both in real time and by way of preestablished compensation control signals. For example, the proposals described in Fletcher U.S. Pat. No. 3,764,933 and Carpenter U.S. Pat. No. 3,931,586 are real time systems that measure the deviation in the output frequency from an intended level, and inject a correction voltage into the principal control voltage during the oscillator sweep. Disadvantageously, real time control techniques suffer from a lack of speed, i.e. they limit the sweep speed of the VCO, thereby restricting their application and making them unattractive for optical signalling applications (not enough resolution can be obtained if the VCO is swept too fast).
Another linearizing proposal, as described in Neal et al U.S. Pat. No. 4,129,832, involves an operator-controlled calibration technique in which stepwise compensation or correction of the VCO is carried out during a test sweep mode by observing the degree of offset of the VCO output from an intended frequency characteristic, and storing respective correction signals to be used for reestablishing the output of the VCO at the proper frequency. The drawback to this approach is the "staircase" linearization employed, which leads to unacceptable variations in sweep output, so that conventional smoothing techniques for achieving the required output accuracy would result in a slow VCO sweep response; also, operator judgement during correction measurements leads to an additional unforseeable error.
For further illustrations of techniques for VCO linearization that embody conventional operational principles such as employed in the above-referenced systems, attention may be directed to Jackson U.S. Pat. No. 4,103,250; Borofka et al U.S. Pat. No. 4,038,612; Martin U.S. Pat. No. 3,504,294; and Weber U.S. Pat. No. 4,109,807.